The present invention relates to an ultrasonic oscillation circuit for generating a driving signal for an ultrasonic vibrating element.
FIG. 1 shows general characteristics of an ultrasonic vibrating element near a resonance frequency, i.e. characteristics of an impedance and a phase difference between the voltage applied across the element and the current flowing through the element. The phase difference becomes zero at two frequency points P.sub.1 and P.sub.2 and the impedance becomes minimum at the lower frequency point P.sub.1, but becomes maximum at the higher frequency point P.sub.2. It is matter of cause that in order to transmit the electric power to the ultrasonic vibrating element in an efficient manner, it is advantageous to vibrate the element at the lower frequency point P.sub.1. Therefore, the lower frequency point is also called the optimum frequency point.
In order to drive the ultrasonic vibrating element at the above mentioned optimum frequency point P.sub.1, there have been proposed various ultrasonic oscillation circuits in, for instance, Japanese Patent Publications Kokai Sho 48-79,613, Sho 52-27,625, Sho 56-7,669 and Sho 59-87,078, Japanese Utility Model Publication Kokai Sho 59-180,774, and PCT International Publication W087/01276. In these known ultrasonic oscillation circuits, the phase difference between the voltage applied across the ultrasonic vibrating element and the current flowing through the element is detected and the oscillation frequency is controlled in accordance with the phase difference such that the phase difference falls within a predetermined narrow range.
However, in the known ultrasonic oscillation circuits, since the oscillation frequency is controlled in accordance only with the phase difference between the voltage and the current of the driving signal, even if the phase difference becomes equal to a given value e.g. zero, the element might vibrate at the higher frequency point P.sub.2 at which the impedance becomes maximum.
Further, the actual resonance frequency of the ultrasonic vibrating element is influenced not only by the resonance frequency of the element itself, but also by a mechanical resonance point of an ultrasonic wave propagating medium which is brought into contact with the element, so that there are plural frequency points at which the phase difference has the same value. This results in that the resonance frequency of the ultrasonic vibrating element is not well controlled within a desired frequency range only by the phase difference.
In the ultrasonic oscillation circuit disclosed in the above mentioned PCT International Publication W087/01276, there is provided a phase control loop in which a phase difference between an output signal from a voltage controlled oscillator and a driving signal for the ultrasonic vibrating element is detected and a control voltage corresponding to the detected phase difference is applied to the voltage controlled oscillator by means of a limiter. In such a circuit, when the impedance of the ultrasonic vibrating element is changed to a large extent, the control voltage having an amplitude equal to a threshold voltage of the limiter is continuously applied to the voltage controlled oscillator, and thus the oscillation frequency of the voltage controlled oscillator could not be returned into the optimum frequency point P.sub.1 promptly.